If you stay at the Legoland Hotel you can enter the park an hour early, which can be good for some attractions. You also are right there and don't have to worry about parking.
But you can enter through a special back entrance if you stay at MarBrisa so you don't have to pay parking that way either.
You have a chance for an ocean view at MarBrisa and not at Legoland hotel.
MarBrisa has multiple pools and a fitness area and is obviously a timeshare so you can do meals there. They also have in-room laundry which is HUGE for traveling with small children.
Legoland hotel has rooms that are Lego themed and very cute. And I believe you get free buffet breakfast.
You can get a discount at the Legoland hotel if you are a passholder, but beware there are restrictions and blackout dates.
The beware at MarBrisa is the sales presentation.
There is also a new hotel onsite at Legoland - Castle Hotel. It's brand new and is set to open next month.
https://www.legoland.com/california/about-us/press/press-room/legoland-castle-hotel-opening/